A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost converter is one example of a switched-mode power converter that is typically employed in off-line applications wherein power factor correction at the input and a stable, regulated voltage at the output are desired.
A non-isolated boost converter may form a portion of a power factor corrector and generally includes a boost inductor and a power switch coupled to the boost inductor. The boost converter further includes a boost diode coupled to a node between the boost inductor and the power switch. The boost converter still further includes an output capacitor coupled across an output of the boost converter. The output capacitor is usually large to ensure a constant output voltage. A load is then connected in parallel across the output capacitor. The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
The boost converter generally operates as follows. The power switch is closed (conducting) for a first interval (D interval). The boost diode is reverse-biased, isolating the output capacitor and, therefore, the load from the input of the boost converter. During this interval, the input voltage supplies energy to charge the boost inductor and the boost inductor current increases. Since the load is isolated from the input voltage, a stored charge in the output capacitor powers the load. Then, for a second interval (1-D interval), the power switch is opened (non-conducting). The boost inductor current decreases as energy from both the boost inductor and the input flows forward through the boost diode to charge the output capacitor and power the load. By varying a duty cycle of the power switch, the output voltage of the boost converter may be controlled.
As previously mentioned, the boost converter, when employed in a power factor corrector, generally provides adequate power factor correction. The power factor is defined as a ratio of the actual power delivered to the load to a product of the root mean square values of the voltage and current at the input of the power factor corrector. The conventional boost converter, however, cannot directly process the AC power available from the AC line. The power factor corrector, therefore, includes an input full wave rectifier bridge to rectify the AC voltage from the AC line. The rectified AC voltage may then be processed by the boost converter. The rectifier bridge is subject to dissipative losses, particularly at low AC line voltages (e.g., 85 to 100 VAC). Further, the rectifier bridge may contribute to electromagnetic interference noise generated by the power factor corrector.
Analogous to other types of power supplies, the power factor corrector is subject to further inefficiencies that impair its overall performance. More specifically, the power switch and boost diode may be subject to conduction losses that reduce the efficiency of the power factor corrector. Additionally, the power switch (e.g., field-effect transistor) is subject to switching losses that occur, in part, when a charge built up in a parasitic capacitance of the power switch is dissipated when the power switch is transitioned to a conducting state. Furthermore, the boost diode may also be subject to a reverse recovery phenomenon, when the power switch is conducting, that induces a substantial current spike through both the power switch and the boost diode. The losses associated with the power switch and the boost diode increase as the switching frequency of the power factor corrector increases. Therefore, the reverse recovery and switching losses associated with the boost diode and power switch will impair the overall efficiency of the power factor corrector.
In accordance therewith, the power factor corrector is subject to electromagnetic interference associated with a switching noise and a ripple current therein. The ripple current generally occurs when a volts-second across the boost inductor changes. The switching noise generally occurs when a current or voltage of the power factor corrector is switched leading to, for instance, the power switch transitioning to a conducting state. Thus, in the environment of the boost converter topology, as the power switch transitions between conducting and nonconducting states, the power factor corrector suffers from a ripple current and switching noise. Relatively speaking, the switching noise may contribute more severely to the electromagnetic interference than the ripple current condition. Therefore, the higher the switching frequency, the higher the potential for the deleterious effects associated with the electromagnetic interference and the power losses, which will also impair the overall efficiency of the power factor corrector.
Accordingly, what is needed in the art is a controller for a power factor corrector and a method of regulating the power factor corrector that addresses and resolves the deleterious conditions that detract from an operation thereof.